Laser systems are employed in a variety of applications including communications, medicine, and micromachining. These applications utilize a variety of laser wavelengths and output powers. In particular, high power laser beams having an ultraviolet (UV) wavelengths are widely used. Currently, there is no commercially available gain medium that directly generates UV laser beams. Thus, UV laser beams are typically generated through nonlinear processes, such as harmonic generation.
Two such harmonic generation configurations include an extracavity harmonic generation configuration and an intracavity harmonic generation configuration. The extracavity configuration generates harmonics outside of a resonant laser cavity. In other words, a laser beam is generated in a resonant laser cavity and directed to a crystal positioned external to the cavity. The intracavity configuration generates harmonics inside of a resonant laser cavity, which is generally more efficient than an extracavity configuration.
Generating a laser beam that is a third or higher harmonic of a fundamental frequency entails generating first a laser beam that is a second harmonic of the fundamental frequency. The extracavity and intracavity configurations generally do not convert all of the second harmonic beam to a third or higher order harmonic beam. Thus, the unused portion of the second harmonic beam reduces the overall efficiency of the laser system and reduces the power of the resultant third or higher order harmonic beam.
FIG. 1 is a schematic diagram of a known intracavity configuration for generating a third harmonic laser beam. A laser 100 employs a laser medium 102 positioned along an optical path 104 of a laser cavity 106 formed by end mirrors 108 and 110, optical pumping input couplers 112 and 114, and an output coupler 116. Laser 100 is pumped with two laser diode pumps 118 and 120. An optical fiber 122 directs laser radiation generated by laser diode pump 118 into laser cavity 106 through optical pumping input coupler 114. Likewise, an optical fiber 124 directs laser radiation generated by laser diode pump 120 into laser cavity 106 through optical pumping input coupler 112. A Q-switch 126, such as an acousto-optic Q-switch (AO-QS), is positioned along optical path 104 and is driven at an appropriate pulse repetition rate (PRR) to obtain short energetic pulses from laser 100. As a laser beam 130 having a fundamental wavelength resonates within cavity 106 between end mirrors 108 and 110, laser medium 102 amplifies laser beam 130.
A second harmonic generation (SHG) crystal 140 is positioned along optical path 104. As laser beam 130 passes through SHG crystal 140, SHG crystal 140 generates a second harmonic laser beam 142 having half the wavelength of laser beam 130. As laser beam 130 and second harmonic laser beam 142 pass through a third harmonic generation (THG) crystal 150, which is also positioned along optical path 104, THG crystal 150 generates a third harmonic laser beam 152 having one-third the wavelength of laser beam 130. Although second harmonic laser beam 142 reflects off end mirror 110, a portion of second harmonic laser beam 142 that is not used in generating third harmonic laser beam 152 exits cavity 106 as an unused, wasted second harmonic laser beam 144 via output coupler 116. Wasting unused second harmonic laser beam 144 lowers the conversion efficiency from the fundamental harmonic to the third harmonic and lowers the total power of third harmonic laser beam 152 that might otherwise be obtained. Third harmonic laser beam 152, which has the desired wavelength (e.g., a UV wavelength of 355 nm), exits cavity 106 as an output laser beam 154 via output coupler 116. Thus, output coupler 116 is highly reflective for laser beam 130 and antireflective for second harmonic laser beam 142 and third harmonic laser beam 152. Laser beams 130, 142 and 152 are shown axially offset from one another for illustration purposes.
U.S. Pat. No. 5,943,351 of Zhou et al. describes one attempt to improve the efficiency of generating a third or higher harmonic beam from a second harmonic beam. As shown in FIG. 2, a laser 200 includes a cavity having a first mirror 210, a lasing rod 220, an acousto-optic Q-switch 222, a second mirror 250, a SHG crystal 230, a third mirror 252, a THG crystal 232, and a fourth mirror 254. A main cavity is formed by mirrors 210 and 254, which cause oscillation of a fundamental beam 212 at 1064 nm using a Nd:YAG rod 220. Mirrors 250 and 254 form a first sub-cavity for the intracavity second harmonic generation (i.e., 532 nm) to create a second harmonic beam 214 therein, and mirrors 252 and 254 form a second sub-cavity for the third harmonic generation (i.e., 355 nm) to create a third harmonic beam 216.
Use of mirror 250 results in a significantly deteriorated third or higher order harmonic beam quality because a beam mode of second harmonic beam 214 is different each time second harmonic beam 214 passes through THG crystal 232. For example, after second harmonic beam 214 is generated by SHG crystal 230, second harmonic beam 214 converges as it propagates from SHG crystal 230 toward mirror 254 (assuming fourth mirror 254 is coincident with a beam waist). As second harmonic beam 214 passes through THG crystal 232, a portion of second harmonic beam 214 will be used to generate third harmonic beam 216 and a portion of second harmonic beam 214 will remain unused. After the unused portion of second harmonic beam 214 reflects off mirror 254, the unused portion of second harmonic beam 214 diverges as it propagates from mirror 254 toward mirror 250. After the unused portion of second harmonic beam 214 reflects off mirror 250, the unused portion of second harmonic beam 214 keeps diverging in a direction toward mirror 254 and continues to diverge as it subsequently propagates between mirrors 250 and 254 (assuming mirror 250 is a flat mirror). Thus, each time the unused portion of second harmonic beam 214 passes through THG crystal 232, the beam mode (e.g., beam radius and beam divergence) of the unused portion of second harmonic beam 214 will be different, which results in a significantly deteriorated third or higher order harmonic beam quality and conversion efficiency. Accordingly, as noted in column 9, lines 6-9 of Zhou, the beam quality factor (i.e., M2 factor) of the third or higher order harmonic beam is 1.6, which is likely larger than what could be achieved without recycling the unused portion of second harmonic beam 214. As a point of reference, a diffraction-limited Gaussian beam has an M2 factor of 1.0.
Thus, the present inventors have recognized a need for a system and method for recycling an unused portion of an intermediate harmonic beam (e.g., a second harmonic beam) to improve higher order harmonic beam generation efficiency (e.g., third or higher order harmonic beam generation efficiency) without sacrificing higher order harmonic beam quality.